- Remove ifdi_tx_intr_enable from API, devices that have one can handle
  the flow of credit updates
- Handle multi-fragment packets
- Use the offset of the next fragment and state in the software
  descriptor to indicate where we are in a packet (this allows us
  to handle discontiguous descriptors within a logical packet)
- add isc_txd_credits_update for devices that have no explicit
  notification of tx credits updates